Extraction of plutonium values from organic solutions



2,918,349 Patented Dec. 22, 19 59 EXTRACTION OF PLUTONIUM VALUES FROMORGANIC SOLUTIONS Glenn T. Seaborg, Chicago, Ill., assignor to theUnited States of America as represented by the United States AtomicEnergy Commission N Drawing, Application October 12, 1945 Serial No.622,098

7 Claims. (Cl. 23-145) The present invention relates to the solventextraction of heavy metal compounds, and more particularly to .theextraction of a compound of a transuranic element from an organicsolvent solution thereof by means of subjecting the latter to extractionwith an aqueous solution in the presence of an acid medium.

.An object 'of this invention is to provide a method for the separationand purification of a transuranic element contained in an organicsolvent solution together with other lighter element impurities.

Another object of this invention is to provide a process for theextraction of plutonium from the aforesaid organic solvent solution bymeans of an aqueous solution in the presence of an acid medium whereby asubstantial separation of said light element impurities from theplutonium is effected.

Other objects of this invention will be apparent from the descriptionwhich follows:

It is known that plutonium can be produced in small quantities by thebombardment of natural uranium with neutrons. The designation plutoniumor element 94 as used throughout the present description refers to thetransuranic element having an atomic number of 94. The expression 94means the isotope of element 94 has an atomic weight or mass of 239.Similarly, the terms element 93 or Np refer to the new element known asneptunium having an atomic number of 93.

Uranium is composed of three isotopes, namely, U U 5, and U the latterbeing present in excess of 99 percent of the whole. When U is subjectedto the action of slow or thermal neutrons, a fourth isotope, U isproduced which has a half-life of 23 minutes and undergoes beta decay toNp which decays further by beta radiation with a half-life of 2.3 daysto yield plutonium. In addition to the formation of the transuranicelements, neptunium and plutonium, there are simultaneously pro- .ducedother elements of lower atomic weight known as fission fragments. Thesefission fragments are composed of two distinct element groups, i.e., alight and heavy element group. The light group contains elements havingatomic numbers of between about 35 and 46 while the heavier group iscomposed of elements of atomic numbers varying between about 51 and 60.The elements of these groups as originally produced are considerablyovermassed and undercharged, and hence are highly unstable. By betaradiation, however, they quickly transform themselves into isotopes ofthese various elements having longer half-lives. The fission fragmentsand their resulting decay products are commonly known as fissionproducts.

The various radioactive fission products have half-lives ranging from afraction ofa second to thousands of years. Those having very shorthalf-lives may be substantially eliminated by ageing the material for areasonable period before handling. Those with very long half-lives donot have sufficiently intense radiation to endanger personnel protectedby moderate shielding. On the other hand, the

fission products having half-lives ranging from a few days to a fewyears have dangerously intense'radiations which 2 cannot be eliminatedby ageing for practical storage periods. These products are chieflyradioactive isotopes'of Sr, Y, Zr, Cb, and R11 of the light group andTe, l, Cs,

Ba, La, Ce, and Pr of the heavy group.

It may be readily seen that plutonium produced as generally set forthabove is contaminated with considerable quantities of uranium andfission products. In fact, the plutonium constitutes only a very minorportion of the irradiated mass, i.e., generally less than one percentthereof. In view of such a low concentration of plutonium in theirradiated metal, it becomes apparent that the procedure employed torecover that element must be highly efficient in order to be at allpracticable.

There have been devised a number of procedures for the removal andconcentration of plutonium from'extremely dilute solutions thereof. Ingeneral, such methods involve the formation of various insolublecompounds in said dilute solutions capable of carrying plutonium in thereduced state. The carrier precipitate and plutonium thus obtained arethen dissolved and the plutonium is oxidized to PuO in which state ofoxidation it is soluble in the presence of said carrier. Under theseconditions, the plutonium remains in solution and the fission productsare removed when the carrier is added. Thereafter, the dissolvedplutonium is reduced to a valence state in which it is carriable by theaforesaid carrier and removed from solution in the form of a carrierprecipitate which may again be dissolved and the plutonium purifiedfurther if considered necessary or desirable by repeating the abovecycle. This procedure, however, is obviously cumbersome andtime-consuming, since it requires a number of such steps in order toeffect a substantial removal of fission products and other impurities.

In copending application-United States Serial No. 481,- 660, filed April3, 1943, and patented as U.S. Patent No. 2,811,415 on October 29, 1957,a process for separating plutonium compounds from aqueous nitric acidsolutions, aqueous hydrochloric acid solutions, or other suitableaqueous media is described and claimed and essentially involves theextraction of said compounds from such aqueous solutions by suitableorganic solvents. Such a process can be utilized for the separation ofelement '94 from aqueous solutions in any phase of the separationprocedure previously described for the recovery of said element fromneutron-irradiated uranium. While this procedure serves to effectivelyrecover the plutonium from the original solution and at the same time toseparate that element from various other impurities, no substantialdecrease is etfected in the percentage of certain types of fissionproducts associated therewith and hence separation of element 94 fromthe organic solvent by distillation would-result in the procurement ofan impure product.

By the present invention it has been discovered that plutonium compoundsmay be readily and effectively separated fro-m organic solvent solutionsof the type obtained in the aforesaid copending application containingsaid compounds together with fission products. By sub jecting suchsolutions to extraction in this manner, it has been found, plutonium ofrelatively high purity is obtainable inasmuch as the fission productswhich have been extracted with the plutonium as well as otherundesirable impurities tend to remain behind in the organic solventphase, particularly when a limited amount of acid aqueous medium is usedfor the extraction. In such a process, the plutonium may reenter theaqueous solution in the trivalent, tetravalent or hexavalent state.

Recovery of element 94 from its original aqueous solution may likewisebe effected by first subjecting the solut on containing ionic plutoniumin its tetravalent or hexavalent state to extraction with a suitableorganic solvent and thereafter re-extracting the resulting organicsolvent solution with an aqueous solution containing a suitable reducingagent such as, for example, sulphur dioxide, hydrazine, hydroxylamine,or the like. In em ploying this specific embodiment of the presentinvention, it has been observed, on contact of the aqueous solution withthe. organic solvent, element 94 is reduced to its trivalent statethereby becoming insoluble in said organic solvent and passing into theaqueous phase. Also during this operation, particularly where hexavalentplutonium is originally present, a portion of the latter at least may bereduced to the tetravalent state and pass into the aqueous phase whereit is reduced further to the trivalent state. By this general procedurea substantially quantitative recovery of the plutonium present in theorganic solvent is effected, while at the same time the quantity ofimpurities associated with the plutonium is materially reduced.

Likewise, it will be apparent that organic solvent solutions of element94, which have been obtained from the original aqueous plutoniumsolution containing a suitable salting-out agent, may be subjected toextraction with an aqueous solution of a reducing agent of the generaltype specified above to secure an aqueous solution of substantially pureplutonium.

The aqueous solutions from which plutonium may be extracted inaccordance with the present invention suitably contain plutonium of atleast +4 valence state, said ions being substantially free from strongcomplexing agents. Preferably, no ion should be present in the aqueoussolution which forms ionic complexes with plutonlum more strongly thandoes the nitrate ion. The

.hydroxyl ion, and the anions of acids which ionizein aqueous solutionsto a substantially smaller degree than nitric acid, are particularlydisadvantageous from the standpoint of complexing tetravalent plutonium.Thus, hydroxyl, sulphate, phosphate, fluoride, and oxalate ions tend tocomplex tetravalent plutonium sufficiently to decrease itsextractability into organic solvents. It is therefore desirable tominimize the concentrations of free ions of this class in the aqueous ororganic solvent solutions to be extracted. Most of the complexing ionscan be excluded in the preparation of the aqueous solutions.Alternatively, an interfering ion may itself be complexed by anotherion, or its concentration as a free ion may otherwise be minimized. bycontrol of the concentration of another ion which can combine with it.Thus, the fluoride ion can be complexed by zirconyl ion, and thehydroxyl ion can be suppressed by hydrogen ion, to

reduce their interference with the extraction of tetravalent plutonium.

The extraction. of hexavalent plutonium is relatively free frominterference by hydroxyl ion. Hexavalent plutonium can be extracted fromaqueous solutions of any hydrogen ion concentration suflicient toprevent the precipitation of a basic plutonium compound. An aqueoussolution of plutonyl nitrate may thus be extracted without the necessityof free nitric acid in the solution. In the case of tetravalentplutonium, on the other hand, it is desirable to maintain free acid inthe aqueous solution. Solutions of plutonous nitrate for extraction withorganic solvents should have a pH not substantially above 2.5, andpreferably should have a concentration f free nitric acid of at least 1N.

In order to improve the transfer of plutonium from the original aqueoussolution to the organic phase, it is generally desirable to incorporatea salting-out agent in the aqueous solution. A salting-out agent, forthis purpose, has the same characteristics as a salting-out agent forpreviously known solvent extraction processes, i.e., high solubility inthe solution to be extracted and low solubility in. the extract phase.The preferred saltingout agents for use in the present invention arethose having a common ion with the compound being extracted. Thus, if anitrate of plutonium is being extracted, the salttng-out agent ispreferably an inorganic nitrate, Ex-

NaNO Ca( a)2 KNO 3)z LiNO Mg( 3)2 NH NO a)s Mn(NO t m The concentrationof the salting-out agent which is desirable in any particular case willdepend on the valence of the cation and the concentration of the commonanion due to any free acid in the solution. In the case of 1 N nitricacid solutions, it is desirable to employ a concentration of'a univalentnitrate of at least 3 M, and preferably 5-10 M. Equivalentconcentrations of polyvalent nitrates may be employed at the same acldconcentration, and the salt concentration may suitably be increased ordecreased with decrease or increase in acid concentration.

The resulting organic solvent solution of element 94 and fissionproducts is then extracted with an acidifiedaqueous solution, as aresult of which element 94 passes into the aqueous phase leaving asubstantial portion of the associated impurities in the organic solvent.Suttable acids for use in such solutions arenitric acid, hydrochloricacid, perchloric acid and the like.

. If desired, extraction of tetravalent plutonium from its solution inan organic solvent may be facilitated by use of a complexing agentcapable of complexing tetravalent plutonium in the aqueous solution. Forexample, the organic solution containing tetravalent plutonium may beextracted with an aqueous medium contalnmg sulphate or oxalate ions orother anion of an acid which is more weakly ionized than nitric acid.Where the plutonium is in the hexavalent state reducing agents capableof reducing plutonium to tetravalent or trivalent state such as sodiumnitrite, ferrous ions, bromite ion may be incorporated.

In carrying out the process of this invention, previously knownextraction procedures and apparatus may be em- I ployed. The extractionmay be elfected by batch, contom of the column and a suitable means forwithdrawing the aqueous acid solution employed in re-extraction. Withcertain obvious changes, solvents heavier than water can be employedwith substantially equally good results.

The plutonium may be recovered from the aqueous extract phase by anysuitable procedure such as evaporation of the aqueous acid solution,crystallization with an isomorphous crystalline carrier, or adsorptionon a solid adsorbent,

It is apparent that the procedures described above may be applied to theextraction of plutonium from solutions containing various impurities oflower atomic weight which have partition coefficients between organicsolvent and aqueous phases lower than that of plutonium. Thepurification effected by this procedure is especially advantageous inseparating plutonium from the fission products contained in solutionsderived from neutron-irradiated uranium. Repeated extractions with oneor more types of acidified aqueous solutions may be used to effectpurification and concentration of plutonium to an extent sufficient toenable final recovery of a pure compound of plutonium.

The process may be applied to extraction of plutonium from water oraqueous medium by various solvents which at? Substantially immisciblewith the. aqueous solution to be extracted and which contain at least 1atom capable of donating an electron pair to a coordination bond. Suchsolvents suitably comprise water-immiscible organic compounds containingan oxygen, sulphur, or nitrogen electron-donor atom. It will be evident,however, that most nitrogen-containing organic compounds of this typeare basic in nature and will be unsuitable for the extraction ofplutonium from acidic aqueous solutions. Such compounds may be used, ifdesired, to extract substantially neutral solutions containing smallamounts of hexavalent plutonium. Most organic solvents containing oxygenor sulphur donor atoms may be used for the extraction of acidic aqueoussolutions as well as neutral solutions, and the oxygenated organicsolvents are the preferred extractants for use in the present process.

Although most normally liquid organic compounds containing an atom withexcess electrons, such as oxygen, sulphur, or nitrogen, are capable offorming a coordination bo-nd, it will be evident to those skilled in theart that certain molecular structures can interfere with thiselectron-donating property. Electron-attracting constituents such ashalogen atoms can off-set the electron donating property of an atom suchas oxygen, if present in s ufiicient number and proper relationship tothe donor atom. For this reason, it is preferable to employ compoundscontaining only carbon, hydrogen and electrondonor atoms. It will alsobe apparent that certain molecular configurations can give rise tosteric hindrance which may interfere sufiiciently to prevent theformation of coordination bonds. Tertiary carbon atoms adjacent anelectron-donor atom and long chains of nondonor atoms linked to a donoratom are especially undesirable in this respect. The preferred solventsare those in which the donor atom is linked to a hydrogen atom or tonon-tertiary carbon atoms and in which at least one component linked tothe donor atom contains less than four consecutive non-donor atoms.

The following are examples of suitable solvents for use in the presentprocess:

Ethyl ether Z-phenoxyethanol Z-benzyloxyethanol 1,2-diethoxyethanel-ethoxy-Z-butoxyethane Methylisobutylcarbinol Methyl ethyl ketoneMethyl amyl ketone Methyl isobutyl ketone Mesityl oxide 2-ethylbutanolThe examples which follow are illustrative of various modifications ofthe process of the present invention and demonstrate the adaptability ofsuch a procedure to the recovery of plutonium from organic solventsolutions which contain the impurities normally associated with thatelement.

EXAMPLE I A solution of 5 N in ammonium nitrate and 3 N in nitric acid,which contained 1.09 milligrams/ml. of plutonium, 6.2 milligrams/ml. ofzirconium, and 6.2 milligrams/ml. of lanthanum, was agitated with anequal volume of methyl isobutyl ketone for a period of approximately tenminutes after which the ketone layer was separated, washed with an equalvolume of a solution 5 N in ammonium nitrate and 3 N in nitric acid inorder to assist in the removal of fission products and other impurities,and then subjected to extraction with a volume of water which wasapproximately 0.75

6 percent of the volume of ketoneemployed. "The'orig'i-nal feed solutionwas subjected to three such extraction cycles and the resulting aqueousextract was analyzed-for :plutonium content on the basis of itsgalpharadiation and that of the original feed solution. The plutonium contentof the extract was found to be 83.8 percent of that in h initial u ion ss ro a i is an l s s indicate the product thus obtained to be 99.75percent plutonium and 0.25 percent zirconium. Lanthanum and otherimpurities commonly associated with element 94 were not present indetectable amounts.

EXAMPLE II A continuous batch extraction was carried out withplutonium-containing solutions in which the plutonium content in eachinstance was 150;]. g. The plutonium was first extracted from eachaqueous solution which was 10 N in ammonium nitrate and 1 N innitric'acid, by means of the various organic solvents indicated below.The solvent was introduced at the bottom of an extraction column, havinga capacity of 15 ml., through a fritted glass disc and rose through thecolumn of product solution to the top where it was withdrawn. The,resulting sol vent solution was then subjected to re-extraction byintroducing the same at the bottom of a second column of the samecapacity and containing 1 N nitric acid, in a similar manner. In bothinstances the organic solvent was forced through the aqueous solution inthe extraction and re-extraction columns continuously by displacementfrom a storage column using water under a constant pressure head.

It will be apparent, of course, that on recycling the organic solvent asubstantially quantitative separation of the plutonium from the originalsolutio'n can be effected.

EXAMPLE III A SO-gram sample of uranyl nitrate hexahydrate prepared fromneutron-bombarded uranium was melted and sufiicient quantities of sodiumdichromate and nitric acid to render the mixture 0.2 M and 0.5 N inthese two respective reagents were added to oxidize the plutonium to itshexavalent state. The resulting solution which contained uranyl nitrate,pluto'nyl nitrate, and fission products was agitated with diethyl ether,and more than percent of the plutonium, substantially all of the uraniumand about one percent of the fission products dissolved in the etherlayer of the resulting two-phase systern. The ether layer was thenshaken with small portions of water saturated with sulphur dioxide inorder to reduce and extract the plutonium. The combined water po'rtionscontained only a'small fraction of the original uranium and about 90percent of the plutonium.

EXAMPLE IV The table below demonstrates the ability of the process ofthe present invention to separate plutonium from impurities which arecommonly associated therewith. The extraction cycles employed consistedof shaking with an aqueo'us solution 10 N in ammonium nitrate, 1 N innitric acid, and containing a known amount of the particular lightelement impurity studied with an equal volume of solvent. The solventwas then washed with an contact withan aqueous nitric acid solution inthe absence of a reducing agent and separating the resulting extract andratfinate phases.

2. The process of claim 1 in which the organic solvent 5 ismethylisobutyl ketone.

' equal volume of 1 N nitric acid, and the nitric acid solution wasthereafter analyzed for the given element to determine the separationfactor, which is that fraction of theimpurity originally present thatwas extracted and back-extracted together with plutonium.

general, it may be said that the use of any equivalent or vmodificationof procedure which would naturally occur to those skilled in the art isincluded in the scope of this invention.

This is a continuation-in-part of copending United States application,Serial No. 481,660 filed April 13, 1943, and-patented as US. Patent No.2,811,415 on October 29, 1957, and all subject matter therein, not

inconsistent with the subject matter herein, is incorporated in thepresent disclosure by reference.

What is claimed is:

1. A process for the separation of plutonium from a substantiallywater-immiscible organic solvent solution containing ionic plutonium inan oxidation state of at least +4 which comprises bringing the plutoniuminto Table II Separation factors for light element impurities-Multipliedby 1,000 Solvent Al B Be Cs Fe K Li Na Mg P 131 La. Ru Zr Ethyl ether 20. 07 7 0. 5 1. 0 0. 2 1, 5 0, 5 Methylisobutyl ketone- 0 1 0 3 0. 2 1.0 0. 2 0. 1 0. 4 2 0. 4 ethyl-n-amyl ketone-.- 0 2 0 3 1.0 2 1.0 0.7 0.62 2 2-ethylbuty1 Cellosolvc.-- 0 2 1. 5 0.3 0.2 0.2 0.5 0.2Triglycoldichlorlde--- 2 0. 25 0. 08 4 4 0 3 0. 1 1. 0 1.0Acetophenone.-. 1 3 0.3 0. 3 3 3 0. 3 0. 3 0. 7 Menthone 2 0.001 5 0. 10, 2 Cyclohexanone 0. 05 1. 0 5 0. 2 2 10 6 0. 2 Ethylbutyl Cellosolv 50. 2 0. 5 0. 5 0, 2 Dlethyl Cellosolve 1 0. 3 5 0. 5 10 6 0. 5Dlmethyltetrahydrofurane. 2 1. 0 5 0. 5 0, 3 Nltromethaue 2 0. 2 10 6 20, 5 Nltroethane.- 5 0. 7 10 4 10 Nltrobenzene 0. 01 3 0. 5 0. 2 4 0. 22 Ethyl sulfide. 2 3 5 2 3 4 5 Trlchloroethylene 0. 1 0. l 5 0- 7 6 3 43. The process of claim 1 in which the organic solvent is ethyl ether.

4. The process of claim 1 in which the organic solvent is methyl-n-amylketone. a i i 5. The process of claim 1 in which the organic solvent isZ-ethylbutyl Cellosolve. I

6. The process of claim 1' in which the nitric acid solution has aconcentration of about 1 N.

7. In a process for the separation of plutonium from light elementimpurities commonly associated therewith in a solution of asubstantially water-immiscible organic solvent, the steps which comprisecontacting the solution in the absence of a reducing agent with anaqueous inorganic acid solution containing a common anion saltingoutagent with regard to the plutonium salt to be extracted, and separatingthe organic solvent thus treated.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS FOR THE SEPARATION OF PLUTONMIUM FROM A SUBSTANTIALLYWATER-IMMISCIBLE ORGANIC SOLVENT SOLUTION CONTAINING IONIC PLUTONIUM INAN OXIDATION STATE OF AT LEAST +4 WHICH COMPRISES BRINGING THE PLUTONIUMINTO CONTACT WITH AN AQUEOUS NITRIC ACID SOLUTION IN THE ABSENCE OF AREDUCING AGENT AND SEPARATING THE RESULTING EXTRACT AND RAFFINATEPHASES.